Not many children do that, but it is certainly not a rarity. The amazing thing is not that it occasionally happens but that it doesn’t always happen. Consider this scenario: hold up a bottle or cup and say to your son, “Do you want some milk?” If you get a “yes” or a nod, you might say, “OK, I will get you some.” You have called your son “you” and yourself “I.” That sort of inversion goes on all the time in conversation: “Come here to me”; “Go find your shoes.” Is it any wonder that your son begs, “Hold you”? Miraculously, we somehow come to realize that “I” and “Me” and “You” are not labels for objects like milk and shoes (or the names of specific people) but are designations relating to role or position. Your son will gradually figure that out and change.
Here is a suggestion that will help the transition. Use the full noun rather than the pronoun in most of your statements. “Let Mommy see it” (rather than “Let me see it”). Does (his name) want to go outside and play (rather than “Do you want to go outside and play?”). After a while, be repetitive and say: “Let Mommy see it; let me see it.”
Finally, I don’t think that sign language had anything to do with his pronoun reversal. I consider it an excellent thing to do with young children and encourage you to continue it.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.